Field of the Invention
The present invention is generally in the field of semiconductors, and more particularly, to dynamically adjusting biasing for signal processing circuits.
Background
Portable communication devices, such as cellular telephones, for example, use signal processing circuits to process weak signals. Signal processing circuits, such as low noise amplifiers, for example, are used to amplify weak signals, such as a signal captured by an antenna. As such, low noise amplifiers are often placed in the front end of a receiver circuit in a portable communication device. When using a low noise amplifier, the effect of noise in the receive chain is reduced by the gain of the low noise amplifier, while the noise of the low noise amplifier itself is injected into the receive signal. Thus, the low noise amplifier optimally boosts the signal power while adding as little noise and distortion as possible.
Low noise amplifiers and other signal processing circuits are often implemented in one or more stages of transistors and other related circuitry. In most applications, the operating point of the transistor is set by providing a bias current or voltage to one of the terminals of the transistor. A good low noise amplifier has a very low noise figure and is biased with a low quiescent current. In linear amplifiers, an input signal gives a larger output signal which varies in proportion to the input signal about the bias point without any change in shape. However, because a transistor is a nonlinear device, the transistor amplifier only approximates a linear device. For low distortion, the transistor is biased so that the output signal swing does not drive the transistor into a region of extreme nonlinear operation.
A constant bias current applied to the low noise amplifier can bias the amplifier to operate with a low noise figure and low quiescent current and good linearity when no signal or small signals are present at the input of the amplifier. However, when a large input signal is present, the amplifier can be driven out of the linear mode. This results in greater noise and distortion to the information signal. Radio Frequency (RF) feedback in the RF path can be used to improve the linearity of the amplifier, but this negatively impacts the noise figure and the gain.